Rupert Stuart-Smith, Oxford Climate Society President 2017/18
Since the fairly disastrous 2009 UN climate conference in Copenhagen (COP15), the slogan of many of the countries most vulnerable to the impacts of climate change has been “1.5 to stay alive”. Six years later, the Paris Agreement (and all its signatories) implicitly acknowledged the inadequacy of the 2°C goal agreed in 2009, and recognised that limiting climate change to 1.5°C above pre-industrial levels ‘would significantly reduce the risks and impacts of climate change’. Yet with the global policymaking focus of the past few years firmly centred on limiting climate change to 2°C, relatively few efforts have been made to understand the future emissions permissible if we are to stay within 1.5°C.
In fact, the answer to the question of our ‘carbon budget’ for 1.5°C is perhaps more dependent on what story you want to tell than the predictive abilities of climate modellers. Depending on what baseline of ‘pre-industrial’ temperatures is used, it is possible to come up with very different (but equally scientifically accurate) estimates of the extent of past manmade climate change.
Millar et al. (2017) use a baseline of average global temperatures from 1861-1880 (equivalent to that used by the Intergovernmental Panel on Climate Change), and calculate that global temperatures have risen 0.93°C from the baseline to the present decade (when the influence of short-term variability from El Niño is excluded). Based on their projections, only a further 200 Gigatonnes of carbon can be added to the atmosphere between now and 2055, and by that point we must have reduced our net carbon emissions (the balance of release into the atmosphere and removal from it) to zero, if we are to have a reasonable chance of limiting climate change to 1.5°C. To put this into context, current annual net CO2 emissions are approximately 10.4 GtC, leaving less than 20 years of emissions at current rates to exceed the 1.5°C carbon budget identified by this paper.
Figure 1: Idealised mitigation trajectory for limiting climate change to 1.5°C. Dashed line shows a peaking of global emissions in 2020, followed by linear decline to net zero in 2055. Thin solid orange line is the climate response to the emissions scenario in 66% of CMIP5 models, showing a warming of under 1.5°C. Thick solid line shows the 50th percentile of the climate response to this scenario. Source: https://www.carbonbrief.org/guest-post-why-the-one-point-five-warming-limit-is-not-yet-a-geophysical-impossibility.
Based on Millar et al., to stay within the 1.5°C budget, unwavering commitment to rapid global decarbonisation is required, and emission reductions of 4-6% per year will be needed in the 2030s and 2040s. This rate of CO2 emission reductions is historically unprecedented and requires worldwide replacement of existing capital. This will include a revolution in our energy systems through the development and massive deployment of renewables, and an as-yet undemonstrated scale of Carbon Capture and Storage (CCS) and CO2 Removal (CDR), for instance through Bioenergy with CCS (BECCS). The rate at which this transition can happen is limited due to the lifespans of existing infrastructure and the inertia of our economic system, and Millar et al. advocate for immediate and ambitious emission reductions to have any chance of keeping up with the precipitous fall in emissions demanded by their 1.5°C scenarios.
At the same time, the key message of Millar et al. is that the 1.5°C temperature goal is not impossibly ambitious, even though current national pledges require dramatic strengthening. With current national commitments falling far short of global ambition on climate change mitigation, technologically and economically feasible scenarios such as those this paper propose can provide a strong framing for the global stocktake process to increase national level climate commitments.
Alternative estimates of the future emissions compatible with limiting climate change to 1.5°C, using different measures of global temperature and baseline periods (such as 1750, since when global temperatures have risen more than they have since the mid-nineteenth century), indicate far smaller emission budgets. Under some estimates, cumulative emissions committing us to 1.5°C will be reached as soon as 2021.
The question is: is the bigger the panic the better? Does presenting climate change mitigation in line with the 1.5°C goal as being an unsurmountable challenge propel individuals, companies and world leaders into action? Or rather, in line with Miller et al., are we more likely to be able to inspire the behavioural, economic and policy revolutions required if science explains that we have a chance to avoid the vast human suffering brought about by inadequate climate change mitigation and warming of over 1.5°C, but only if we act now?
Join Professor Myles Allen, a co-author of the Millar et al. paper, the Oxford Climate Society, Oxford Martin School and Environmental Change Institute in a discussion on the findings of the paper and these questions on Sunday at COP23, in an event chaired by Kya Raina Lal.
Full details: https://www.oxfordmartin.ox.ac.uk/event/2514
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